A dispensing system for a liquid such as motor vehicle fuel generally consists of means for dispensing said liquid, essentially comprising distributors fitted with pumps designed to deliver the fuel from a storage tank to the tank of the vehicles at a liquid flow rate Q.sub.L. The distributors also have a system for measuring the liquid, connected to a pulse generator enabling a computer to establish the volume and price of the fuel delivered, which are shown in text on a display with which the distributors are fitted.
When provided with a means for recovering the emitted hydrocarbon vapors, said system has recovery means designed to deliver said vapor at a vapour flow rate Q.sub.v through a passage from the tank of the vehicles to a collection tank, for example, the storage tank, the vapor flow rate Q.sub.v is controlled by a value g characteristic of said recovery means in order to maintain a ratio of proportionality Q.sub.v =k Q.sub.L between the vapor flow rate Q.sub.v and the liquid flow rate Q.sub.L where k equals or is close to 1. Finally, a measuring means will enable the vapor flow rate Q.sub.v to be determined.
More often than not, said recovery means consists of a pump which sucks the vapor from the tank in order to deliver them to the hydrocarbon storage tank. This being the case, the characteristic value g will be the rotation speed of said pump, which is controlled by the pulse generator of the distribution means.
However, in the majority of cases, it is not possible to impose on the pump in a simple manner a speed which is proportional to the liquid flow rate Q.sub.L.
In practice, the operating conditions can vary considerably from one system to another due to losses in pressure on the recovery passage, upstream and downstream of the pump.
There may be calibrated valves on a level with the collection tank which can cause a pressure therein that is different from atmospheric pressure, corresponding to an additional loss of pressure which is imposed on the pump in the recovery passage.
There may be an internal leakage in the recovery pump, dependent upon the upstream-downstream pressure differential which affects its efficiency.
In order to obtain a given vapor flow rate Q.sub.v, a rotation speed must be imposed on the recovery pump and this rotation speed depends on the system.
In order to take account of the parameters mentioned above, it is common practice to calibrate the entire system when it is installed on the site. During this calibration procedure, a speed is set for the recovery pump and the corresponding vapor flow rate Q.sub.v is measured using a flow meter or gas counter. Accordingly, a ratio is established between the speed and the vapor flow rate Q.sub.v by taking a sufficient number of measurements to define the characteristic of the pump under these operating conditions. This ratio is stored in memory in a micro-processor.
During normal operation, the flow meter is shut off and when hydrocarbons are being dispensed at a liquid flow rate Q.sub.L, the micro-processor searches the memory for the speed to be imposed on the recovery pump so that Q.sub.v =Q.sub.L.
However, this known recovery method has the following disadvantages.
Pressure losses may occur on the recycling passage over time as a result of gradual partial blocking due to dust, and the change in the section of elastomer pipes due to the prolonged presence of hydrocarbons. This is particularly prevalent in the part of the passage located upstream of the pump, which is generally provided in the form of an elastomer tube surrounded by pressurised liquid, this part representing the core of a coaxial flex pipe.
Internal leakage which might develop in the pump due to wear. As is the case with vane pumps, the density of the vapor which will vary depending on the hydrocarbons and the temperature of the vehicle tanks as the ambient temperature changes, thereby altering the effect upstream and downstream pressure losses will have.
The vapor pressure in the collection tank may also vary depending on the hydrocarbons and the temperature.
Consequently, the technical problem to be resolved by the invention is that of proposing a method of recovering vapor emitted when liquid is being dispensed to a tank with the aid of a system comprising:
means for dispensing the liquid, designed to deliver said liquid at a liquid flow rate Q.sub.L from a storage tank to said tank, PA1 means for measuring said liquid flow rate Q.sub.L, PA1 means for recovering vapor, designed to deliver said vapor at a vapour flow rate Q.sub.v from the tank to a collection tank, said vapor flow rate Q.sub.v being controlled by a value g characteristic of said recovery means, and PA1 means for measuring said vapor flow rate Q.sub.v. PA1 the recovery means are initially calibrated by air suction by varying said value g and by measuring, for each value g.sup.0, of g, the corresponding vapor flow rate Q.sub.vi of air in order to build an initial calibration table T.sub.0 : EQU T.sub.0 =[g.sup.0.sub.i,Q.sub.vi ] PA1 with each dispensing of liquid n: PA1 the vapor flow rate Q.sub.v is measured at each time interval, PA1 a coefficient K.sub.n of similarity is calculated on the basis of the differences between the measured values of Q.sub.L and Q.sub.v, and PA1 a new calibration table T.sub.n is set up in readiness for the next dispensing operation n+1 by means of: EQU T.sub.n =[g.sup.n.sub.i,Q.sub.vi ]=K.sub.n .multidot.T.sub.g. PA1 during said initial calibration step, an initial correlation table Ho is set up linking the vapor flow rate Q.sub.v to the flow rate Q of vapor indicated by the flow meter (123): EQU [H.sub.0 =[Q.sup.0.sub.i,Q.sub.vi ] PA1 during the liquid dispensing operation n:
This method takes account of the gradual change in characteristic parameters of the vapor as it is fed along the recovery passage, enabling a time-delayed recalibration of the characteristic value g to be performed as a function of the measured vapour flow rate Q.sub.v.